Evaporation of sulphite waste liquor



C. J. LOCKMAN EVAPORATION OF SULPHITE WASTE LIQUOR March 26, 1957 Filed March 27, 1953 Fig.1

CAkL J'oHA/v L Oct/(MAN United States Patent O EVAPORATION OF SULPHITE WASTE LIQUOR Carl Johan Lockman, Stockholm, Sweden, assignor to Aktiebolaget Rosenblads Patenter, Stockholm, Sweden, a corporation of Sweden Application March 27, 1953, Serial No. 344,986

1 Claim. (Cl. 159-47) This invention relates to the evaporation of sulphite waste liquor in a multiple stage or multiple effect system to a concentration which is suitable for the combustion of said liquor, i. e. at least to a content of solids of about 50%.

, Recently it has been experienced both experimentally and in practice in such evaporation that the scale formers precipitated from the liquor, in first line calcium sulphate and calcium sulphite, have a very small tendency to deposit as scales in the evaporation device or devices in which the content of solids in the liquor is very high while in comparable conditions the formation of scales is very heavy when the content of solids in the liquor is low. In certain cases it has been found that at a content of solids of 50% in the liquor the formation of scales is practically eliminated and at a solid content of at least 20 to 25% the scale formation as a rule is so moderate that the evaporation bodies might be cleaned intermittently in an economical way. At lower contents of solids and down to a content of about at which the liquor as a rule is available for evaporation, however, the danger for a very troublesome incrustation is very great.

The main object of the present invention is to eliminate or reduce the said difliculties in connection with incrustation. A further object of the invention is to conduct the liquor in such a way that already when contacting the heating surfaces of the evaporation system for the first time the liquor shall have such a concentration that probably the scales formed may be removed in an economical manner, even if the fresh liquor to be evaporated be introduced into the system as a solid content which is so low that the formation of such hardly removable scales cannot be avoided without certain steps being taken.

With these and other objects in view the invention comprises the combination that (a) the supply of the liquor to be evaporated in the system, is divided into shunted streams to two or more of the stages or effects of the system and to such a number of the stages that the liquor in said stages will reach a solid content in excess of and that (b) in each of the effects into which the liquor is thus introduced the liquor concentrated in the stage in question is caused to circulate vigorously over the heating surfaces of that stage whereby when the liquor to be evaporated enters said stage and at the same time is mixed with the circulating liquor the solid content of this mixture will not fall below 20%.

The invention will be more particularly described with reference to the accompanying drawing in which:

Figures 1 to 3 each show diagrammatically one of three different ways of passing the liquor through a four stage evaporating system.

In the figures all details referring to design of apparatus or to the way of passing heating medium and condensate in the evaporator are omitted since the method according to the invention may be exercised quite independently of such details, which may be designed in "ice any suitable manner known to persons skilled in the art.

Fig. 3 refers to the invention while Figures 1 and 2, drawn for the sake of comparison, illustrate how methods, known in the art, are performed. In all figures the heating surfaces of the separate stages are shown diagrammatically by means of rectangles 1, 2, 3 and 4 respectively, and the passage of the liquor is indicated by solid lines, the flowing direction being indicated by arrows. In all cases an amount G of liquor is evaporated from 10 to 50% by weight of solids under the assumption that the heating surfaces and the rates of evaporation respectively are equally large in all stages. All numerical statements of percentage relate to weight of solids.

According to Fig. l the whole amount of the thin liquor to be evaporated enters heating surface 1 and on passing the separate heating surfaces in succession from surfaces 1 to 4, it will leave the separate heating surfaces concentrated to 12.5%, 16.6%, and 50% respectively. Only on having passed the half of the heating surface 3 the liquor has obtained a concentration exceeding the critical 20% and thus slightly over 60% of the total heating surface of the evaporator will be in contact with liquor of lower concentration and, consequently such heating surface is liable to be covered by scale. These parts of the heating surface are indicated by hatching to give a clear view of the magnitude of the problem.

Fig. 2 shows the effect of introducing the first step of the combination according to the invention only. Here /3 of the thin liquor is fed to heating surface 1 and then to surfaces 2, 3 and 4 in succession, While /3 of the liquor is supplied to heating surface 2 and then to surfaces 3 and 4 in succession and, finally, /3 of the liquor is supplied to heating surface 3 and then to surface 4. The obvious result is that the liquor will leave the two first heating surfaces concentrated to a value well above 20%, namely to 25%. The advantage thereof is, however, not very great in itself since the liquor still does not reach the critical concentration of 20% on the first heating surface without first having to pass about thereof and on the second heating surface not without having to pass about 70% thereof. On the third and fourth heating surfaces 3 and 4 the circumstances remain as in Fig. 1. Thus, still 50% of the total heating surface is liable to be covered by scale distributed as indicated by the hatched parts of the rectangles.

According to my invention, however, the higher concentration obtained by arranging for shunted feeding of the liquor as now indicated is combined with a recirculation of concentrated liquor from the outlet end to the inlet end of the heating surface at a sufficiently large rate so as to produce at the inlet end a mixture of recirculated and freshly supplied liquor of a concentration not below 20%. In these circumstances it is necessary that the concentration of the out-going liquor exceeds 20% at each heating surface. This effect is achieved by shunting the total amount of in-going thin liquor appropriately divided to an appropriate number of stages according to the circumstances.

Fig. 3 illustrates the effect of utilizing all steps according to the invention, including the first step according to Fig. 2 in combination with the second step of recirculation.

As in Fig. 2 the amounts and concentrations of the fresh supplied liquor to each of the first three heating surfaces are G/3.10%, 0.46 G, 14.2% and 0.6 G, 16.6%, respectively and the outgoing liquor is from all three surfaces 25%. Such liquor is recirculated at all three surfaces at a ratio, calculated to fresh ingoing liquor, of 4:1; 3:1; and 2:1 at surfaces 1, 2 and 3 respectively with the result that all heating surfaces are contacted byliq'uor at-a' concentration of at least 22%, that is to say over the critical 20%. No part of the total heating surface is contacted by liquor of lower concentration.

Fi-g ssu asa one oi many-possible: ways of performitiglthe' inventionr In other circumstances it may be ncessaryuonsu-itahle to modify the schemeifor instance as to the number. and selection of parallel feed stages, the proportions of the' liquor quantities supplied to each stage and the rate of circulation. Once the method disclosed, suclf modifications should, however, be judged 1:0 be-within the scope of the invention as they may resuitv from mere mathematical calculationsl within the capability of any pers'onskilledainthe art.

What .I claim-is:

A method of reducing the in'crustation difiiculti'es in evaporating s'ulphite waste liquor to a content of solids amounting to at least about fifty percent in a multiple stage system, comprising in combination the steps of introducing thing liquor to' be evaporated into the systemin a plurality of shunted streams, simultaneously distributed to substantially the same plurality of evaporation stages, passing concentrated liquor from stage to stage of said plurality of stages therebyintroducing such liquor from such stage into the subsequent stage mixed with thin liquor introduced into said subsequent stage, drawing off concentrated liquor from each of said plurality of stages and mixing said drawn off concentrated liquor from each of said stages with the said mixed incoming liquor for the same stage and at a rate to form a mixture having a solidcontent of at least twenty percent, and heating the total of liquor introduced into each stage of said plurality of stages to concentrate that liquorin each of the stages into a solid content in excessof twenty percent.

References Cited inthe file of this patent UNITED STATES PATENTS 

